5
5 The Integumentary System
Skin (Integument)
Consists of three major regions
Epidermis—superficial region
Dermis—middle region
Hypodermis (superficial fascia)—deepest region
Subcutaneous layer deep to skin (not technically part of skin)
Mostly adipose tissue
Epidermis
Keratinized stratified squamous epithelium
Cells of epidermis
Keratinocytes—produce fibrous protein keratin
Melanocytes
10–25% of cells in lower epidermis
Produce pigment melanin
Epidermal dendritic (Langerhans) cells—macrophages that help activate immune system
Tactile (Merkel) cells—touch receptors
Layers of the Epidermis: Stratum Basale (Basal Layer)
Deepest epidermal layer firmly attached to the dermis
Single row of stem cells
Also called stratum germinativum: cells undergo rapid division
Journey from basal layer to surface
Takes 25–45 days
Layers of the Epidermis: Stratum Spinosum (Prickly Layer)
Cells contain a weblike system of intermediate prekeratin filaments attached to desmosomes
Abundant melanin granules and dendritic cells
Layers of the Epidermis: Stratum Granulosum (Granular Layer)
Thin; three to five cell layers in which the cells flatten
Keratohyaline and lamellated granules accumulate
Layers of the Epidermis: Stratum Lucidum (Clear Layer)
In thick skin
Thin, transparent band superficial to the stratum granulosum
A few rows of flat, dead keratinocytes
Layers of the Epidermis: Stratum Corneum (Horny Layer)
20–30 rows of dead, flat, keratinized membranous sacs
Three-quarters of the epidermal thickness
Functions
Protects from abrasion and penetration
Waterproofs
Barrier against biological, chemical, and physical assaults
Dermis
Strong, flexible connective tissue
Cells include fibroblasts, macrophages, and occasionally mast cells and white blood cells
Two layers:
Papillary
Reticular
Layers of the Dermis: Papillary Layer
Papillary layer
Areolar connective tissue with collagen and elastic fibers and blood vessels
Dermal papillae contain:
Capillary loops
Meissner’s corpuscles
Free nerve endings
Layers of the Dermis: Reticular Layer
Reticular layer
~80% of the thickness of dermis
Collagen fibers provide strength and resiliency
Elastic fibers provide stretch-recoil properties
Skin Markings: Friction Ridges
Epidermal ridges lie atop deeper dermal papillary ridges to form friction ridges of fingerprints
Skin Markings: Cleavage Lines
Collagen fibers arranged in bundles form cleavage (tension) lines
Incisions made parallel to cleavage lines heal more readily
Skin Color
Three pigments contribute to skin color:
Melanin
Yellow to reddish-brown to black, responsible for dark skin colors
Produced in melanocytes; migrates to keratinocytes where it forms “pigment shields” for nuclei
Freckles and pigmented moles
Local accumulations of melanin
Skin Color
Carotene
Yellow to orange, most obvious in the palms and soles
Hemoglobin
Responsible for the pinkish hue of skin
Appendages of the Skin
Derivatives of the epidermis
Sweat glands
Oil glands
Hairs and hair follicles
Nails
Sweat Glands
Two main types of sweat (sudoriferous) glands
Eccrine (merocrine) sweat glands—abundant on palms, soles, and forehead
Sweat: 99% water, NaCl, vitamin C, antibodies, dermcidin, metabolic wastes
Ducts connect to pores
Function in thermoregulation
Sweat Glands
Apocrine sweat glands—confined to axillary and anogenital areas
Sebum: sweat + fatty substances and proteins
Ducts connect to hair follicles
Functional from puberty onward (as sexual scent glands?)
Specialized apocrine glands
Ceruminous glands—in external ear canal; secrete cerumen
Mammary glands
Sebaceous (Oil) Glands
Widely distributed
Most develop from hair follicles
Become active at puberty
Sebum
Oily holocrine secretion
Bactericidal
Softens hair and skin
Hair
Functions
Alerting the body to presence of insects on the skin
Guarding the scalp against physical trauma, heat loss, and sunlight
Distribution
Entire surface except palms, soles, lips, nipples, and portions of external genitalia
Hair
Consists of dead keratinized cells
Contains hard keratin; more durable than soft keratin of skin
Hair pigments: melanins (yellow, rust brown, black)
Gray/white hair: decreased melanin production, increased air bubbles in shaft
Hair Follicle
Extends from the epidermal surface into dermis
Two-layered wall: outer connective tissue root sheath, inner epithelial root sheath
Hair bulb: expanded deep end
Hair Follicle
Hair follicle receptor (root hair plexus)
Sensory nerve endings around each hair bulb
Stimulated by bending a hair
Arrector pili
Smooth muscle attached to follicle
Responsible for “goose bumps”
Types of Hair
Vellus—pale, fine body hair of children and adult females
Terminal—coarse, long hair of eyebrows, scalp, axillary, and pubic regions (and face and neck of males)
Types of Hair
Hair Growth
Growth phase (weeks to years) followed by regressive stage and resting phase (1–3 months)
Growth phase varies (6–10 years in scalp, 3–4 months in eyebrows)
Hair Thinning and Baldness
Alopecia—hair thinning in both sexes after age 40
True (frank) baldness
Genetically determined and sex-influenced condition
Male pattern baldness is caused by follicular response to DHT
Structure of a Nail
Scalelike modification of the epidermis on the distal, dorsal surface of fingers and toes
Functions of the Integumentary System
Protection—three types of barriers
Chemical
Low pH secretions (acid mantle) and defensins retard bacterial activity
Functions of the Integumentary System
Physical/mechanical barriers
Keratin and glycolipids block most water and water- soluble substances
Limited penetration of skin by lipid-soluble substances, plant oleoresins (e.g., poison ivy), organic solvents, salts of heavy metals, some drugs
Biological barriers
Dendritic cells, macrophages, and DNA
Functions of the Integumentary System
Body temperature regulation
~500 ml/day of routine insensible perspiration (at normal body temperature)
At elevated temperature, dilation of dermal vessels and increased sweat gland activity (sensible perspirations) cool the body
Cutaneous sensations
Temperature, touch, and pain
Functions of the Integumentary System
Metabolic functions
Synthesis of vitamin D precursor and collagenase
Chemical conversion of carcinogens and some hormones
Blood reservoir—up to 5% of body’s blood volume
Excretion—nitrogenous wastes and salt in sweat
Skin Cancer
Most skin tumors are benign (do not metastasize)
Risk factors
Overexposure to UV radiation
Frequent irritation of the skin
Some skin lotions contain enzymes in liposomes that can fix damaged DNA
Skin Cancer
Three major types:
Basal cell carcinoma
Least malignant, most common
Squamous cell carcinoma
Second most common
Melanoma
Most dangerous
Basal Cell Carcinoma
Stratum basale cells proliferate and slowly invade dermis and hypodermis
Cured by surgical excision in 99% of cases
Squamous Cell Carcinoma
Involves keratinocytes of stratum spinosum
Most common on scalp, ears, lower lip, and hands
Good prognosis if treated by radiation therapy or removed surgically
Melanoma
Involves melanocytes
Highly metastatic and resistant to chemotherapy
Treated by wide surgical excision accompanied by immunotherapy
Melanoma
Characteristics (ABCD rule)
A: Asymmetry; the two sides of the pigmented area do not match
B: Border exhibits indentations
C: Color is black, brown, tan, and sometimes red or blue
D: Diameter is larger than 6 mm (size of a pencil eraser)
Burns
Heat, electricity, radiation, certain chemicals
(
Burn
(tissue damage, denatured protein, cell death)
Immediate threat:
Dehydration and electrolyte imbalance, leading to renal shutdown and circulatory shock
Rule of Nines
Used to estimate the volume of fluid loss from burns
Partial-Thickness Burns
First degree
Epidermal damage only
Localized redness, edema (swelling), and pain
Second degree
Epidermal and upper dermal damage
Blisters appear
Full-Thickness Burns
Third degree
Entire thickness of skin damaged
Gray-white, cherry red, or black
No initial edema or pain (nerve endings destroyed)
Skin grafting usually necessary
Severity of Burns
Critical if:
>25% of the body has second-degree burns
>10% of the body has third-degree burns
Face, hands, or feet bear third-degree burns
Developmental Aspects: Fetal
Ectoderm ( epidermis
Mesoderm ( dermis and hypodermis
Lanugo coat: covering of delicate hairs in 5th and 6th month
Vernix caseosa: sebaceous gland secretion; protects skin of fetus
Developmental Aspects: Adolescent to Adult
Sebaceous gland activity increases
Effects of cumulative environmental assaults show after age 30
Scaling and dermatitis become more common
Developmental Aspects: Old Age
Epidermal replacement slows, skin becomes thin, dry, and itchy
Subcutaneous fat and elasticity decrease, leading to cold intolerance and wrinkles
Increased risk of cancer due to decreased numbers of melanocytes and dendritic cells
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